1. Field of the Invention
This invention relates to turbocharger systems and more particularly relates to turbocharger systems with two turbocharger stages in series.
2. Description of the Related Art
Emissions regulations on internal combustion engines have become quite stringent and now impose significant constraints on engine design. Various features have been added to engines to respond to new emissions regulations, and to provide operational freedom to pursue the competing goals of meeting emissions and maintaining engine performance. Recent engine development cycles are increasingly using advanced turbocharger systems, such as variable geometry turbos and two-turbo systems. Two-turbo systems allow increased charge density by staging air compression. Further, they allow an engine to exhibit both responsiveness with a small, high pressure turbo stage, and high capacity with a large, lower pressure turbo stage.
One drawback of the two stage turbo system is that at fully rated operation, the high pressure (smaller) turbo is not able to accept the entire exhaust flow of the engine or the turbocharger will overspeed. One solution in the current art is to install a bypass line around the high pressure turbo and direct a portion of the exhaust directly to the lower pressure turbo. In many cases, a significant majority of the exhaust is flowing through the bypass line when the engine is operating at fully rated power. It is desirable to have a large flow capacity difference between the high pressure turbo and the low pressure turbo, to maximize the responsiveness of the high pressure turbo while maximizing the flow capacity of the low pressure turbo. However, the greater the flow capacity difference between the high pressure turbo and the low pressure turbo, the larger the bypass valve must be. Therefore, the bypass valve controlling the bypass line is typically large and has a relatively slow response time to go from open to closed. By contrast, the engine is preferably quite responsive. Therefore, in the current art, the engine and/or the turbocharger flow targets must have artificially reduced response to avoid overspeeding the high pressure turbo in transient situations.
Additionally, with a two-stage system in the current art, the high pressure compressor may have a bypass valve because at high inlet air flow rates, the high pressure compressor becomes a restriction in the intake air system. When the compressor bypass valve opens, the compressor then stops absorbing work from the high pressure turbocharger, which can cause instability or overspeed of the high pressure turbocharger, as well as reduce operator satisfaction with inconsistent engine performance.